1. Field of the Invention
The present invention generally relates to the field of high speed fiber optical communications, and more particularly to methods for joining optical fibers with integrated optical waveguides.
2. Technical Background
It is typical to connect an optical fiber to an optical waveguide, for instance, when preparing an integrated optics device for use in a communications system or network. The term xe2x80x9coptical waveguidexe2x80x9d is used herein, in distinction to the expression xe2x80x9coptical fiber,xe2x80x9d to designate a lightguiding medium typically formed on or in a planar (silica) substrate of rectangular cross-section (or a chip as referred to interchangeably herein). Note that the term xe2x80x9cplanar waveguidexe2x80x9d conventionally refers to a unit comprising a core and a cladding region; that is, a light path and the substrate in or on which the light path resides; however, in the present application a distinction will be made between the lightpath (xe2x80x9cwaveguidexe2x80x9d) and the substrate (or chip) when appropriate to avoid confusion. Typically, the light guiding region (core) extends to the edge or extremity of the chip. Waveguides of this type are frequently encountered in integrated optics applications, for example, as components of a multiplexer or demultiplexer or, more generally, as part of an integrated optical circuit.
The conventional fusion technique for connecting optical fibers together makes use of an electric arc discharge. However, this technique is not suitable for connecting an optical fiber to a waveguide due to the geometrical characteristics of the waveguide and the heat capacity of the waveguide which is higher than that of the optical fiber. Accordingly, it has been proposed to connect an optical fiber to a planar type silica waveguide by bringing the elements into abutment and applying a laser beam to cause them to fuse together. However, a further problem arises; namely, a high energy laser beam used to create the fusion joint can cause the waveguide core to bend and, moreover, excessive melting of the optical fiber can occur. On the other hand, if a lower energy laser beam is utilized, then the joint between the waveguide and the optical fiber has less strength than required for deployed applications.
One proposed approach to solving the above problem is to pre-heat the waveguide by means other than the laser beam used for effecting the fusion between the waveguide and the optical fiber. Such pre-heating makes it possible to reduce the power of the laser beam needed to create the fusion connection. However, such an approach complicates the process for forming the fusion joint and, in many cases, requires the use of specialised equipment and/or the modification of the structure of the integrated optical component to be connected to the optical fiber.
Another consideration is the ever increasing demands being placed on optical communications technology which have complicated the hardware and software involved, and placed great emphasis on achieving more efficient manufacturing and deployment. For example, the growth of metro networks and the associated signal routing, add/drop and switching requirements for narrowband Wavelength Division Multiplexer (WDM) systems employing 16, 32, or 40 or more channels now makes it advantageous to be able to connect multiple fibers to respective waveguides (referred to hereinafter as xe2x80x9cmulti-fiber fusion pigtailingxe2x80x9d) with good performance characteristics, accuracy, repeatability, and efficiency rather than single fiber/waveguide connections.
The present invention provides a method for forming an accurate fusion joint between an optical fiber and an optical waveguide in a chip, with low optical losses and a strong joint.
The invention further provides a method for accurately performing multi-fiber fusion pigtailing between multiple fibers and multiple waveguides in a chip, with low optical losses and strong joints.
An embodiment of the present invention provides a method for connecting an optical fiber to an optical waveguide in a chip, including the steps of aligning the optical fiber with the waveguide, bringing the optical fiber and waveguide into abutment, and irradiating a zone of abutment between the optical fiber and the waveguide with a laser beam having a sufficient power and a desired spatial energy distribution; and blocking a portion of the laser beam such that the energy corresponding to a substantially central part of the spatial energy distribution is reduced with respect to the energy corresponding to a peripheral part of the spatial energy distribution in the beam at the zone of abutment.
An aspect of this embodiment of the invention involves interposing a shield element in the path of the laser beam upstream of the zone of abutment in order to eliminate a substantially central part of the laser beam while allowing a peripheral part thereof to pass.
According to another aspect of the invention, the step of reducing the energy in the substantially central part of the spatial energy distribution with respect to the peripheral part is carried out by dividing the laser beam, upstream of the zone of abutment, into several distinct beams, and directing these beams towards the zone of abutment. This can be done, for example, by a splitting mirror disposed in the path of the laser beam upstream of the zone of abutment, with the distinct beams then directed towards the zone of abutment by parabolic mirrors. The distinct beams resulting from the splitting of the laser beam can be slightly defocused at the zone of abutment.
Another aspect of the invention relates to accurately positioning the zone of abutment with respect to the location of a focused or slightly defocused laser beam for fusing the waveguide to the optical fiber. This method aspect involves the steps of obtaining an image of the laser beam on the waveguide chip surface and generating a set of coordinates x1, y1, corresponding to the position of the approximate center of the laser beam. The coordinates x1 and y1 are set off from a border or extremity of the chip by respective amounts xcex94x, xcex94y. A second set of coordinates, x2 and y2, which represent the extremity position of the waveguide, are then determined and the waveguide extremity and abutted fiber are manually or automatically located at a position xcex94X=x2xe2x88x92x1, xcex94Y=y2xe2x88x92y1; that is, the zone of abutment is optimally and accurately positioned in the fusion region of the laser beam. The image of the laser beam on the chip is obtained by a camera which is stationary, along with the laser, relative to the chip and the fiber. Preferably, several images are taken in sequential time order. The positioning method according to the invention provides a positioning accuracy of the zone of abutment of greater than about plus or minus one micron.
In another embodiment of the invention, a method for connecting a plurality of optical fibers to a respective plurality of waveguides in a chip includes the steps of propagating a substantially collimated laser beam through a diffractive optical element (DOE) to simultaneously produce a desired spatial laser energy distribution at a zone of abutment for each of the fiber-waveguide connections.
An aspect of this embodiment provides a method for accurately positioning the energy distributions of the laser beams at each zone of abutment and includes the positioning steps referred to in the positioning aspect described above.
The invention described herein provides a method for strongly, accurately, and efficiently connecting an optical fiber to a waveguide in an optical chip.
In each of the aspects of this embodiment, the energy distribution at the zone of abutment is asymmetrical; that is, the energy impinging upon the waveguide is greater than that which reaches the optical fiber. In this way, the degree of melting of the optical fiber can be controlled.
In another aspect of the embodiment of the invention, a force is applied between the optical fiber and the waveguide in a direction moving the optical fiber and waveguide closer together during the irradiation of the zone of abutment.
In a further aspect, the power cycle of the laser is controlled such that the laser beam power is held at a first, relatively higher level during a first period of time during which the fusion joint is created, and is maintained at a lower level during a second period of time subsequent to the first period of time, allowing gradual cooling of the fusion joint.
The methods of the present invention find application in connecting optical fibers to planar silica waveguides. In order to ensure absorption of the laser energy by the waveguide material, the irradiation step is performed using a laser beam of wavelength greater than 4 xcexcm. Suitable lasers include a Carbon Monoxide (CO) laser having a wavelength of 9.8 xcexcm, and a Carbon Dioxide (CO2) laser having a wavelength of 10.6 xcexcm. Use of a CO2 laser currently provides a cost advantage.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.